1. Field of Invention
The present invention relates in general to ink jet printing, and methods for smoothing the appearance of ink jet prints. In particular, the present invention relates to methods for smoothing the appearance of ink jet prints to provide photorealistic quality prints on a variety of print substrates.
2. Description of Related Art
Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal printers, have at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in at least one channel, or preferably in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or a resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in one of the plurality of channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice, followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink, to form a droplet moving in a direction away from the channel orifice and towards the recording medium. When the ink droplet hits the recording medium, a drop or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type printer, a partial-width-array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage, which is reciprocated to print one swath of information (having a width equal to the length of a column of nozzles) at a time on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion of the swath, so that the next printed swath is contiguous or overlapping with the previously printed swath. This procedure is repeated until the entire page is printed. In contrast, the page-width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Many liquid inks, and particularly those used in thermal ink jet printing, include a colorant or dye and a liquid, which is typically an aqueous liquid vehicle, such as water, and/or a low vapor pressure solvent. The ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid component is removed from the ink and the paper to fix the colorant to the substrate by either natural air drying or by active drying. In natural air drying, the liquid component of the ink deposited on the substrate is allowed to evaporate and to penetrate into the substrate naturally without mechanical assistance. In active drying, the recording medium is exposed to heat energy of various types, which can include infrared heating, conductive heating and heating by microwave energy.
These and other designs of ink jet printers have found use in printing single color images (e.g., typically black-and-white images) as well as multi-color and full-color images. Recently, such printers have also found increased use in printing photorealistic, photo-quality images.
Full color printing has become a desired goal of office, as well as home, products. One type of full color printer that has significant potential for fulfilling such a goal is the ink jet printer, due primarily to its low purchase costs combined with its high quality output. Color printing is accomplished by providing multiple layers or separations of ink on the page. Commonly, colors are provided by subtractive combinations of cyan, magenta and yellow inks. To print black, a combination of equal amounts of cyan, magenta and yellow is printed, or a fourth black ink can be used as a substitute. Undercolor removal, a well known process in the printing arts, can be used to print a single layer of black ink as a substitute for the combination of equal amounts of cyan, magenta and yellow. For a fuller discussion of under color removal and its application to electronically derived or created images, reference is made to J. A. C. Yule, Principles of Color Reproduction, (John Wiley & Sons, Inc., New York, 1967), pages 294-327. Other full color printing processes may use dry powder or liquid toners.
A problem of ink jet printers, as well as with other types of pixel-addressed printers, is that there remains a need to improve the print quality of the printer. For example, as the use of full-color printing increases, there is an increasing demand for more photorealistic quality prints. This demand exists both in the context of providing high quality prints on specially treated or coated papers, as well as on ordinary office or uncoated paper.
Various approaches have been taken in the art to address this demand and provide high quality prints. For example, various particular ink sets have been developed, various specially coated papers have been developed, and various printing methods have been developed, each in an effort to increase the final print quality of the printers.
U.S. Pat. No. 5,917,509 provides increased print quality by controlling power pulses sent to the multiple liquid ink emitters so as to maximize the number of emitters that can be simultaneously energized while keeping the instantaneous power usage within prescribed boundaries. The multiple emitters are organized into banks of emitters whose numbers are small enough that all emitters within a bank can receive a correct level of power simultaneously without exceeding capacity of a shared power source. A circuit interleaves the power pulses to the emitters so that no bank of emitters are receiving power at the same instant of time.
Many patents have addressed the problem of print quality by providing specifically designed inks and ink sets. For example, U.S. Pat. Nos. 5,989,325 and 5,931,995, the entire disclosures of which are incorporated herein by reference, provide such specific ink sets for use in ink jet printers. In particular, U.S. Pat. No. 5,989,325 provides a nonaqueous ink composition comprised of a vehicle, colorant, and a hydrophobic gelling component. U.S. Pat. No. 5,931,995 provides an ink comprised of (1) a liquid aldehyde, a liquid acid, or mixtures thereof; (2) a solid additive aldehyde compound, a solid additive acid compound, or mixtures thereof; (3) a lightfastness UV absorber; (4) a lightfastness antioxidant; and (5) a colorant. In a similar manner, specific ink sets for improved print quality are provided in, for example, U.S. Pat. Nos. 5,858,075, 5,788,754, and 5,738,716, the entire disclosures of which are incorporated herein by reference.
An alternate means to improve the print quality of ink jet printers is to address the ink drying characteristics on the print medium itself. For example, U.S. Pat. Nos. 5,805,190 and 5,864,350 provide methods whereby the drying and setting characteristics of the ink are altered.
U.S. Pat. No. 5,805,190 discloses an ink jet printing method that forms an image by ejecting a printing property improving liquid, which improves the printing property of ejected ink, onto a printing medium in advance of ejection of the ink onto the printing medium. Next, the ink is ejected over the printing property improving liquid applied on the printing medium. The ink is ejected onto the printing property improving liquid layer under a condition where a specific relational expression of a thickness of a liquid layer of the printing property improving liquid covering a region of the printing medium to be covered with the ink after impacting the ink thereon, a diameter of the ejected ink and ink ejection speed, and so forth is satisfied. In this method, the ink impacts the liquid layer and contacts the printing property improving liquid over the entire surface, before the ink comes into contact with fibers of the printing medium paper. Upon such contact between the ink and the printing property improving liquid, the dyestuff in the ink is coagulated or made insoluble from the surface of the ink droplet. As a result, the flowability of the ink into the paper is lowered.
Likewise, U.S. Pat. No. 5,864,350 discloses an apparatus and method in which an ink and a separate liquid, which insolubilizes or coagulates a dye contained in the ink, are ejected to perform printing. The patent discloses a method that reduces consumption of the liquid in the case of employing a printing method where one pixel is formed by a plurality of scanning cycles of a head, such as when printing a pixel represented by red print data, a yellow ink and a magenta ink, by ejecting the separate liquid only in the final scanning cycle. In this patent also, the separately ejected liquid acts to coagulate the dye and to bond the dye to the paper.
Despite these various attempts to improve print quality, there remains a need in the art for improved methods to improve the print quality of ink jet and other printers. This need exists despite the improved ink materials themselves, because print quality is still constrained by the printing parameters of the printer itself.
For example, a remaining problem that needs to be addressed is the fact that in ink jet printing, only a small number of colors, usually cyan, magenta and yellow, and optionally black, are used to produce all of the colors of an image. As a result, a small number, and often one, pixel is used to darken or lighten a color. This is especially true in gradients, where one color fades into another. In the middle of such a gradient, there is an area where one color is peppered over another color, forming an image in that area that is objectionable and typically of poor quality.
For example, FIG. 1 represents, in simplified form, a gradient from a light color (here, white) to a dark color (here, black) formed by an ink jet printer. In FIG. 1, the gradient is formed by precisely positioning multiple light color ink droplets 1 and multiple dark color ink droplets 2 on the print surface. Although the figure shows a gradient from light to dark, the central portion of the image, which generally is in a form of a checkerboard pattern of alternating light and dark dots, is of poor quality. Similarly, FIG. 2 shows, in simplified form, a dark color ink droplet 2 surrounded on all sides by light color ink droplets 1.
Thus, a need continues to exist in the art for means to improve the print quality of ink jet printers, as well as other pixel addressable printers, to provide photorealistic images of high print quality.